In a variety of applications it is desirable to apply particles to a substrate in a predetermined pattern. One such application is the formation of patterns of solder alloys on silicon wafers, printed circuit boards, flip chip, and BGA submounts.
International patent application publication WO 2005/033352 (PCT/US2004/022143) (U.S. Pub. no. 2005/0106329) discloses a process of electrokinetic deposition of metal particles such as tin or tin alloy powder as an effective method to form solderable surfaces and structures on electronic parts and components.
In the electrokinetic deposition process the substrate to be coated having masked surfaces and unmasked surfaces is immersed in a bath with electrokinetic toner. A typical bath (toner) for electrokinetic deposition of tin powder consists of solvent (e.g., Isopar), charge director (e.g., barium petronate), and metal particles. To assure the process of electrokinetic deposition a second (counter) electrode is also immersed in the bath and electrical voltage is applied between two electrodes. Under the influence of an applied electrical field the charged powder particles are deposited on the unmasked surfaces of the coated substrate. One requirement for the process to occur is that the surfaces to be coated should be electrically conductive and also electrically connected to the voltage power supply. This requirement limits the application of electrokinetic deposition to substrates having at least one complete conductive layer.